Field of the Invention
The present invention relates to image capturing apparatuses and methods for controlling image capturing apparatuses.
Description of the Related Art
In the related art, the contrast detection scheme is known in automatic focus detection and automatic focus control of an image capturing apparatus as a typical technique of using a flux of light passing through an imaging lens. In this scheme, an image sensor is used as a sensor for focus detection, an output signal of the image sensor is evaluated, and the position of the imaging lens is moved so that the contrast information is maximized, whereby the focus can be adjusted.
However, the contrast information needs to be evaluated each time the imaging lens is moved to a different position. In addition, after the highest contrast has eventually been found as a result of the evaluation, the imaging lens is moved back to the position where the highest contrast is obtained. Therefore, it takes a long time to complete the process. Therefore, it is difficult to perform a high-speed operation.
In order to address such a drawback, a technique has been proposed according to which a phase difference detection function is incorporated into an image sensor, which can therefore also be used as a focus detection element to directly obtain the amount of defocus of an imaging lens.
For example, according to Japanese Patent Laid-Open No. 2010-219958, a pupil division function is provided to a portion of light receiving elements of an image sensor by decentering the sensitive region of the light receiving portion with respect to the optical axis of an on-chip microlens. By arranging these pixels in the image sensor at predetermined intervals, a phase difference detection function is achieved.
For example, according to Japanese Patent Laid-Open No. 2013-106194, a plurality of photoelectric conversion elements, i.e., an A-pixel and a B-pixel, are provided in a pixel corresponding to each microlens of an image sensor, and an A-pixel output and an (A+B)-pixel output are read out. By performing a subtraction process on these two outputs, a B-pixel output is obtained, whereby a phase difference detection function is achieved.
Japanese Patent Laid-Open No. 2010-219958 above discloses, as an example, a proposal that, in so-called rolling shutter drive, reset lines are scanned in normal pixel rows and in rows including phase difference detection pixels, separately. In this case, in the rows including phase difference detection pixels, a phase difference detection pixel and a normal pixel provided in the same row are read out using the same signal reading method in an image sensor.
The phase difference detection pixel cannot be used as an image signal. Therefore, when a normal image is obtained, the phase difference detection pixel is typically dealt with as a defective pixel, and it is necessary to perform an interpolation process using surrounding pixels. Therefore, it is difficult to excessively increase the proportion of phase difference detection pixels arranged. Therefore, although the same reading circuit configuration as that which is used when no phase difference detection pixel is included can be used, the resolution for phase difference detection is low.
On the other hand, in the related-art technique of Japanese Patent Laid-Open No. 2013-106194 above, all pixels include a plurality of photoelectric conversion elements, and therefore, can each be used as a phase difference detection element, whereby the resolution in the horizontal direction is also improved. However, in order to read an (A+B)-signal and an A-signal from the plurality of photoelectric conversion elements of all pixels, the image sensor requires a reading circuit for reading an (A+B)-pixel and a reading circuit for reading an A-pixel. In particular, a double number of storage capacitors, which occupy a relatively large area in an image sensor, are required, and control lines are complicated, and therefore, the area of a circuit around the image sensor is likely to increase.
As indicated by Japanese Patent Laid-Open No. 2013-106194 above, if all image pixels of the image sensor include a plurality of photoelectric conversion elements, the peripheral read circuit has a complicated circuit configuration, and particularly, a circuit element which has a relatively large area, such as a storage capacitor, is required.
Moreover, an arrangement for subtracting the A-signal from the (A+B)-signal to obtain the B-signal is required in the entire system, disadvantageously leading to large circuit size and high cost. Also, the arrangement for reading out the (A+B)-signal and the A-signal (or the B-signal) is simply equivalent to the two times the number of pixels, and therefore, the reading time is two times as long. Thus, the time it takes to write the column circuit increases, and the horizontal transfer time is simply two times as long, and therefore, it is difficult to perform high-speed reading.
Note that if both the (A+B)-signal and the A-signal are read only from particular rows instead of reading both the (A+B)-signal and the A-signal from all pixels, a longer horizontal transfer time is required only for the particular rows. However, when the cycle of horizontal synchronization is changed only for the particular rows, there is a significantly large load on the system.